This invention relates to the non-destructive examination of welds and tubes inside a nuclear reactor pressure vessel. In particular, this invention relates to the examination of the welds attaching the incore housing to the reactor pressure vessel using ultrasonic transducers. This invention also relates to the eddy current surface examination of the incore housing tubes.
In a conventional boiling water reactor, the reactor core comprises a plurality of fuel assemblies arranged in a spaced array and oriented vertically. Each fuel assembly consists of a fuel bundle and its carrying case, called a fuel channel. Fuel assemblies are grouped in sets of four with a control rod interposed between the four assemblies in each set. The control rods contain a neutron absorbing material, and are inserted between the fuel assemblies in varying degrees to control the reactivity of the core. The entire core is immersed in water which serves as a coolant, as well as a neutron moderator. All these are contained in the reactor pressure vessel.
Interdispersed throughout the core between the fuel assemblies are removable dry-tubes, which house incore flux monitors and other instrumentation. Dry-tubes rest on the lower core support plate, and extend to the top guide, at the top of the core. Below the dry-tubes, and extending through the bottom head of the pressure vessel the guide and incore housing tube configurations are welded in place. The guide tubes extend downward from the lower core support plate to the tops of the incore housing tubes, which then extend through the pressure vessel. The bottoms of the guide tubes are welded to the tops of the incore housing tubes, forming a single unit. A reactor will have anywhere from twenty to sixty such tube configurations, depending on its size.
Nuclear reactors constitute extremely hostile environments for manual examination of any kind. First, nuclear reactors have inherently high levels of radioactivity and radioactive contamination. Secondly, most of the reactor pressure vessel internals are inaccessible for almost any kind of manual examination. A classic example of such inaccessibility is the weld attaching the incore housing to the bottom head of a reactor pressure vessel.
The incore housing consists of stainless steel tubes which penetrate the bottom head of the pressure vessel. Attachment welds seal the boundary between the inner surface of the pressure vessel and the incore housing, as well as provide structural support for the incore housing. Any defects in the attachment welds, e.g. cracks, jeopardize the integrity of the pressure system.
Non-destructive examination of the attachment welds is used to verify their integrity or to discover any incipient defects, so necessary repairs can be made before failure occurs. An ultrasonic probe for "seeing" into the weld and surrounding metal is suitable for such an examination. Ultrasonic probes send a beam of sound waves through a region, and flaws (called indications) cause reflections which are detected and analyzed.
Ultrasonic probes have been used in the past to examine welds inside the reactor pressure vessel. Such a probe is described in "Stub Tube Inspection Device", U.S. Pat. No. 4,548,785, issued Oct. 22, 1985. In this patent a scanning tool, i.e. a device for moving the probe around the region of interest, is placed on top of a stub tube, and the probe is moved vertically along the outside of the stub tube during the scan, then rotated and moved vertically again for another scan. This is done with two tranducers, one which "looks" up, and one which "looks" down. On a given vertical sweep the upwards-looking transducer is on during the upsweep, and the downwards-looking transducer is on during the downsweep. Mechanical switches, at the top and bottom of the sweep, switch the transducers' activation states. No attention is paid to the rotational orientation of the transducers with respect to the reactor. This limits information as to the nature of any flaws since the inspected region is not rotationally symmetric.
The nature of weld inspection inside a pressure vessel is such that every situation requires an inspection device specially suited to the particular circumstances involved. For instance, the probe apparatus described in U.S. Pat. No. 4,548,785 can not be used to inspect incore housing welds because the incore housings are not "stubs", but rather part of a continuous tube structure that extends from below the pressure vessel through the bottom, and up to the bottom of the reactor core support plate. This makes it impractical to use any sort of probe external to the tube. The prior method of inspecting these welds involved removing a flange at the bottom of each incore housing tube outside the pressure vessel, and manually inserting a probe up to the weld area. This necessitates violating the pressure boundary of the pressure vessel, and is extremely unsatisfactory because of the the high levels of radiation to which workers are exposed. Workers must wear a "bubble suit" with an external air supply for radiation protection when performing this type of examination. Also, this process is awkward, expensive, and time consuming. Thus it became necessary to develop a new technique for incore housing inspection.